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Method for phosgene stoichiometry control during the synthesis of acid chlorides

IP.com Disclosure Number: IPCOM000249936D
Publication Date: 2017-May-04
Document File: 4 page(s) / 139K

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Abstract

Acid chlorides (e.g. acetyl chloride, chloroacetyl chloride, propionyl chloride, butyryl chloride…) can be produced by reacting carboxylic acids with phosgene in the presence of a dialkylformamide as the catalyst. The active catalyst species is the so-called "Vilsmeier salt", which - being a salt - has a high electrical conductivity. This publication describes how the amount of added phosgene in an acid chloride synthesis process can be controlled in real time by simply using a sensor that continuously measures the electrical conductivity of the reaction medium.

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Method for phosgene stoichiometry control during the synthesis of acid chlorides

Abstract

Acid chlorides (e.g. acetyl chloride, chloroacetyl chloride, propionyl chloride, butyryl chloride…) can be produced by reacting carboxylic acids with phosgene in the presence of a dialkylformamide as the catalyst. The active catalyst species is the so-called “Vilsmeier salt”, which - being a salt - has a high electrical conductivity. This publication describes how the amount of added phosgene in an acid chloride synthesis process can be controlled in real time by simply using a sensor that continuously measures the electrical conductivity of the reaction medium.

1. Vilsmeier-type catalysts

Vilsmeier-type catalysts can be employed for the phosgenation of carboxylic acids to acid

chlorides. Normally these catalysts are not employed in the form of the Vilsmeier salts in the

synthesis of acid chlorides but in form of the corresponding formamides. Upon reaction with

phosgene these formamides then yield the active catalysts, the Vilsmeier salts (R is usually an

alkyl group like methyl, ethyl, propyl or butyl).

A significant difference of the amide and the Vilsmeier salt is their electrical conductivity. As

the Vilsmeier salt is an ionic species it imparts solutions containing it a significantly higher

electrical conductivity when compared to solutions containing only the corresponding

formamide.

2. Phosgenation control in acid chloride syntheses

In the synthesis of acid chlorides (e.g. acetyl chloride, chloroacetyl chloride, propionyl chloride,

3-chloropropionyl chloride, butyryl chloride, isobutyryl chloride, 4-chlorobutyryl chloride,

valeroyl chloride, isovaleroyl chloride, pivalic acid chloride, 5-chlorovaleroyl chloride etc.) the

control of the exact amount of phosgene can be of importance e.g. for minimization of

phosgene consumption or due to safety issues to minimize the concentration of free phosgene

in the reaction mixture, especially at the end of the reaction.

When using formamides as catalyst precursor the control of phosgene can be easily achieved

in real time by an electrical conductivity measurement. This can be shown exemplarily in the

synthesis of chloroacetyl chloride from monochloroacetic acid (MCA) catalyzed by

dimethylformamide (4 wt.-%). The chlorination is catalyzed by the phosgenation product of

dimethylformamide (DMF), the DMF-Vilsmeier salt. As already stated above DMF-Vilsmeier

salt and DMF differ significantly in the electrical conductivity. Depending on the amounts of

phosgene used, the following cases can be distinguished, where n(X) is the number of moles

of substance X fed to the reactor:

a) n(MCA) > n(phosgene) When the chlorination of monochloroacetic acid is run with substoichiometric amounts

of phosgene, the reaction effluent contains the desired product chloroacetyl chloride,

unconverted mono...